Substantial elements of air travel are automated or aided by various electronic systems. For instance, when an aircraft is landing, the pilot is engaged in a number of activities while approaching the runway. These activities may include adjusting flight speed, adjusting a position of the aircraft, extending and locking landing gear, and other activities. To the extent these systems are controlled, aided, or prompted by computerized electronic systems on the aircraft, the systems may rely on an altitude of the aircraft relative to the runway for controlling the timing of system actions. For instance, the aircraft may be set at one speed when 500 feet above the runway and at a second speed when 200 feet above the runway. An example of a method for automating landings based on aircraft altitude is disclosed in U.S. Pat. No. 5,113,346.
Presently, aircraft may utilize a radio altimeter or ‘radio altitude’ techniques for determining an altitude of the aircraft. Radio altitude techniques determine an altitude of the aircraft relative to the ground immediately beneath the aircraft by sending and receiving radio waves directed downward to the ground and measuring the delay or echo upon receipt of the reflected signals at the aircraft. However, since the altitude of the terrain before the runway may be uneven, or many have an upsloping or downsloping terrain, or otherwise not match the altitude of the runway preceding the runway, radio altitude information fed to the aircraft computerized systems for interpretation may not accurately reflect the altitude of the aircraft above the runway.
FIG. 1 is an illustration of a glidepath angle 10 of an aircraft approach for a runway 12. Horizontal dotted lines mark the altitude 500 ft. above 14 the runway 12 and 200 ft. above 16 the runway 12, as well as the nominal pre-runway terrain 18, which is level with the runway 12. A nominal stage 1 alignment trigger 20 shows the location relative to aircraft glidepath angle 10 where activities that should be initiated at 500 ft. would be initiated. However, because radio altitude system is measuring altitude relative to actual terrain 22, the activities that should be initiated at 500 ft may not be initiated until the actual stage 1 alignment trigger 24, more than 200 ft closer to the runway than desirable. Similar differences are shown between the nominal stage 2 alignment trigger 26 and the actual stage 2 alignment trigger 28. The inaccuracy of the radio altitude system can prevent timely execution of pre-landing activities and, further, leave insufficient time between initiation of stage 1 and stage 2 activities.
Moreover, an upsloping pre-runway such as found at the runway facilities in Albuquerque, N. Mex. airport, graphically illustrated in FIG. 1, may further delay the insertion of the initial stage and compresses the amount of time in the approach available for each of the stages to accomplish their required actions. To work properly, the two stage alignment requires enough time between the first stage of alignment and the second stage of alignment for the first stage of alignment to be completed before the second stage of alignment has been inserted.